U.S. Pat. No. 5,999,417 (the '417 patent) and U.S. Pat. No. 7,050,309 (the '309 patent) describe what is here referred to as the “Intermediate Bus Architecture” and “bus converters.” The entire teachings of these patents are incorporated herein by reference.
The Intermediate Bus Architecture (IBA) has become a popular approach for providing multiple output voltages (for loads such as digital circuits) from a single input voltage source. A first dc-dc converter (sometimes called a “bus converter”), usually providing isolation through a transformer, is used to change the source voltage, say 48V, to an intermediate voltage, say 12V. This intermediate voltage is then used as the input to several non-isolated dc-dc converters (sometimes called “P.O.L. converters,” for “point-of-load”) or linear regulators, each of which create a regulated output voltage appropriate for their respective loads.
When the range of the source voltage is narrow enough, the bus converter can be a device that normally does not regulate. It simply isolates and converts the source voltage to the intermediate voltage by virtue of the turns-ratio of its transformer. For instance, it may have a turns-ratio of 4:1, so that a 48V source becomes a 12V intermediate voltage. As the source voltage ranges from 38V to 56V, the intermediate voltage correspondingly ranges from 9V to 14V. This type of bus converter will be referred to herein as a “non-regulated bus converter.”
Since the non-regulated bus converter does not normally regulate (it regulates only during a turn-on or turn-off transient, or during a current-limit condition, and the like), the intermediate voltage displays a “droop” characteristic. By this it is meant that the value of the intermediate voltage decreases as the current flowing out of the bus converter increases. For the example given above, this might make the intermediate voltage range from 8.5V to 14V over the full range of source voltage and bus converter output current.
This variation of the intermediate voltage is acceptable since the P.O.L.'s can typically operate over such a range of their input voltage.
When the range of the source voltage is wider, such as 36V to 75V, or even 36V to 100V, then a different type of bus converter is often used because the non-regulated bus converter would give too much variation in the intermediate voltage for the P.O.L.'s to handle. This second type of bus converter, referred to herein as a “semi-regulated bus converter,” provides regulation (as well as isolation) so that the intermediate voltage does not vary proportionally to the source voltage. To first order it holds the intermediate voltage approximately constant, although it does permit this voltage to droop as the bus converter's output current increases so that some costs might be saved. For this reason, this type of bus converter is referred to as “semi-regulated.” The droop in the intermediate voltage, which might be around 5% to 10% of the nominal voltage as the bus converter's output current ranges from zero to full rated current, is well within the range of what a typical P.O.L. can handle for its input voltage.
A semi-regulated bus converter has a lower level of performance, in terms of efficiency and power handling capability, than the non-regulated bus converter as a result of its design to provide regulation over the full range of the source voltage.